Two of the main problems which occur in hydraulic engineering are the corrosion of metals and the precipitation of scale forming metal salts in both treated and untreated cooling water systems. The corrosion of metals such as steel, aluminum, brass and copper which are commonly found in water systems, is primarily due to dissolved oxygen and carbon dioxide. Materials which remove oxygen such as sodium sulfite or hydrazine are not economical and are technically inadequate. The use of Zn.sup.++ ion, chromates, molybdates, polyphosphates, orthophosphate, and organophosphates in cooling water to form protective films on metal surfaces is common in the industry. Chromates are very efficient corrosion inhibitors, but they are often environmentally undesirable due to well known toxic effects Zn.sup.++ likewise has environmental problems and it also forms low solubility products with orthophosphate, hydroxide and carbonate which can produce sludge and desposits responsible for promoting corrosion. Polyphosphates are not as efficient as chromates and they are unstable in a cooling water environment in which they decompose by hydrolysis to ortho- and pyrophosphates which often form sludge and deposits. Although organophosphonates provide some corrosion protection, they are not nearly as efficient as chromates.
The formation of insoluble scale-forming metal salts such as calcium phosphate, carbonate, and sulfate have also proven harmful to the overall efficiency of water conducting systems, e.g. deposits in lines and heat exchange equipment. Some of the factors that have been found to affect scale formation are temperature, the pH of the system, the alkalinity, the rate of heat transfer and the concentration and types of ions present. Manganese is readily dissolved as the manganous (Mn.sup.++) ion in water that is free of oxygen and may be found in deep well waters at concentrations as high as 2-3 mg/l. Manganese can exist in several forms depending on the oxidation state, pH, bicarbonate-carbonate-OH equilibria, and the presence of other materials. Concentrations greater than about 0.05 mg/l have been reported to cause troublesome manganese deposition. Concentrations less than this however, can also cause problems by accumulating in a distribution system and then being released in higher concentrations at a later time if a change in the environment should occur, e.g. changes in pH, CO.sub.2 content or alkalinity. Exposure to an oxidizing environment (aeration, chlorination) also may result in manganese deposition. Thus, for example, when low levels of manganese exist in cooling tower feed water, deposition can occur which subsequently results in serious corrosion problems.
Several classes of compounds have been used to control metal ions, e.g. aminocarboxylic acids are of particular importance in water treatment. U.S. Pat. No. 2,396,938 teaches the use of aminocarboxylic acids for treating a boiler both to remove scale and to prevent its formation. A method of simultaneously controlling ion concentration and pH using aminocarboxylic acids is disclosed in U.S. Pat. No. 2,961,311. Commercially available aminocarboxylic acids include nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). These aminocarboxylic acids function by incorporating the metal ion into a ring structure, i.e. by chelation. Metal ions are usually controlled by aminocarboxylic acids on a stoichiometric basis.
The use of methylenephosphonic acid substituted alkylenepolyamines for metal ion control at less than stoichiometric amounts was suggested in U.S. Pat. No. 2,609,390. Later water dispersible polymeric amine chelating agents, which included alkylene phosphonate derivatives, were indicated as having "threshold" effects in scale inhibition applications in U.S. Pat. No. 3,331,773. This term is used to describe the use of the chelating agent in less than stoichiometric amounts. The diamine and polyamine methylenephosphonic acid derivatives are taught and claimed in U.S. Pat. Nos. 3,336,221 and 3,434,969, respectively. Some of the products disclosed in these two patents are available commercially and are recommended as threshold scale inhibitors for alkaline earth metal compounds such as alkaline earth metal carbonates, sulfates, and oxalates. Other patents which disclose heterocyclic nitrogen containing compounds which are useful as chelating agents and may be employed in threshold amounts to control the precipitation of alkaline earth metal salts are U.S. Pat. Nos. 3,674,804; 3,720,498; 3,743,603; 3,859,211 and 3,954,761. In a more recent patent, U.S. Pat. No. 4,229,294, the use of threshold amounts of amino compounds containing both phosphonic and hydroxypropylenesulfonic acid groups for alkaline earth metal scale inhibition is disclosed. The use of aminophosphonates to prevent corrosion of metals in water systems is disclosed by one of the inventors of this application in U.S. Pat. No. 4,640,818 in which such compounds are used in combination with them manganese ion. It is also shown in the '818 patent that in the absence of manganese, phosphonates such as diethylenetriaminepenta(methylenephosphonic acid) (DTPMP) are deleterious to copper or brass in water systems.
The stabilization of soluble manganese to prevent its precipitation is disclosed in U.S. Pat. No. 4,552,665 by the addition of a copolymer of acrylic or methacrylic acid with 2-acrylamido-2-methylpropylsulfonic acid or its methacrylamido analog. This copolymer is also said to be effective in combination with any water soluble polycarboxylate, polyaminophosphonate or phosphate.
In addition, certain non-nitrogen phosphonic acid derivatives of aliphatic acids are useful ion control agents. These can be prepared by reacting phosphorous acid with acid anhydrides or acid chlorides, e.g. the phosphonic derivatives of the anhydrides or chlorides of acetic, propionic and valeric acids, correspond to the formula ##STR1## wherein R is a lower alkyl radical having 1 to 5 carbon atoms. The method of making and use of these products is described in U.S. Pat. No. 3,214,454. The use of threshold amounts to prevent calcium precipitation is disclosed and claimed therein. Representative of this type of product is the commercially available 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP).